A dynamic random access memory (DRAM) stores data by supply of electric charge to a capacitor. Thus, the off-state current of a transistor for controlling the supply of electric charge to the capacitor is preferably small. This is because the smaller the off-state current of the transistor is, the longer the data holding period can be; thus, the frequency of refresh operations can be reduced. Patent Document 1 discloses a semiconductor device that can hold stored content for a long time by using a transistor including an oxide semiconductor film and having extremely small off-state current.